A hot-dip Al—Zn alloy coated steel sheet such as a hot-dip Al—Zn alloy coated steel sheet containing 25 mass % to 90 mass % of Al in the zinc or zinc alloy coating layer show better anti-corrosion property than a hot-dip galvanized steel sheet.
Generally, the hot-dip Al—Zn alloy coated steel sheet is produced using as the base steel sheet, a thin steel sheet obtained by hot rolling or cold rolling a steel slab, and subjecting said base steel sheet to recrystallization annealing and hot-dip coating treatment in an annealing furnace of a continuous galvanizing line (CGL). The hot-dip Al—Zn alloy coating layer formed comprises an alloy layer existing in the interface with the base steel sheet, and an upper layer existing thereon. Further, the upper layer is mainly composed of a part where Zn is contained in a supersaturated state and Al is solidified by dendrite solidification (α-Al phase), and a remaining interdendritic part (Zn rich phase), and the part where Al is solidified by dendrite solidification is laminated in the thickness direction of the hot-dip coating layer. Due to the characteristic coating structure of the upper layer, the course of corrosion development from surfaces becomes complicated, and corrosion less likely reaches the base steel sheet. As a result, the hot-dip Al—Zn alloy coated steel sheet has better anti-corrosion property than a hot-dip galvanized steel sheet with a hot-dip coating layer with the same thickness.
The molten bath contains incidental impurity, and Fe leaching for example from steel sheets or equipment in the molten bath, and in addition, Si for inhibiting excessive growth of the alloy layer, is normally added. Si exists in the alloy layer, in the form of an intermetallic compound, or in the upper layer in the form of an intermetallic compound, solid solution, or a simple substance. Si provides an effect of inhibiting growth of the alloy layer of the interface of the base steel sheet of the hot-dip Al—Zn alloy coated steel sheet, and the thickness of the alloy layer is around 1 μm to 5 μm. If the thickness of the hot-dip coating layer is the same, the thinner the alloy layer is, the upper layer which provides an effect of improving anti-corrosion property is thicker, and therefore, limiting growth of the alloy layer contributes to the improvement of anti-corrosion property. Further, the alloy layer is harder than the upper layer and acts as the origin of cracks during processing. Therefore, limiting the growth of the alloy layer would reduce generation of cracks and provide an effect of improving bending workability. Further, in the generated cracks, the base steel sheet is exposed and anti-corrosion property is poor. Therefore, inhibiting growth of the alloy layer and inhibiting generation of cracks would improve the anti-corrosion property in parts subjected to bending.
Because of their excellent anti-corrosion property, a demand for hot-dip Al—Zn alloy coated steel sheets is increasing mainly in the field of building materials such as roofs, walls which are exposed to outdoor environments for a long period of time. Further, hot-dip Al—Zn alloy coated steel sheets are recently being used also in the field of automobiles. Particularly in the field of automobiles, there is a demand, as one of the countermeasures for global warming, to reduce the weight of the vehicle body to improve fuel efficiency and reduce CO2 emission. For this reason, there is a strong demand for weight reduction by using high strength steel sheets, and gauge reduction by improving anti-corrosion property of steel sheets. However, when using hot-dip Al—Zn alloy coated steel sheets in the field of automobiles, particularly for outer panels, the following problems arise.
Generally, when using hot-dip Al—Zn alloy coated steel sheets as automobile outer panels, the hot-dip coated steel sheets are delivered to automobile manufacturers or the like in a state where hot-dip coating has already been applied on the steel sheets in a continuous galvanizing line, then worked into shapes of panel components, then subjected to chemical conversion treatment, and then three-coat refinishing for automobiles consisting of electrodeposition coating, intercoating, and topcoating. However, with outer panels using hot-dip Al—Zn alloy coated steel sheets, when the coating film is damaged, the hot-dip coating layer with a characteristic phase structure, consisting of two phases i.e. the above described α-Al phase and the Zn rich phase causes preferential dissolution of Zn (selective corrosion of the Zn rich phase) in the interface between the coating film and the hot-dip coating, with the damaged part acting as the origin. This progresses deep into the sound part of the coating film and causes a large coating film blister. As a result, there were cases where sufficient anti-corrosion property (anti-corrosion property after coating) could not be secured for outer panels using hot-dip Al—Zn alloy coated steel sheets.
On the other hand, anti-corrosion property after coating is also a problem when using hot-dip Al—Zn alloy coated steel sheets in the field of building materials as roof material or wall material for buildings. When used as roof materials or wall materials, hot-dip coated steel sheets are normally delivered to construction companies or the like in a state where undercoating and topcoating have already been applied on the steel sheets, and are then sheared into a desired size before use. Therefore, the edge surfaces of the steel sheets without a coating film inevitably becomes exposed, and with these part acting as the origin, coating film blisters referred to as edge creeps may be caused. When using hot-dip Al—Zn alloy coated steel sheets in the field of building materials, similarly to the case for using them as the automobile outer panels, selective corrosion of the Zn rich phase occurs in the interface between the coating film and the hot-dip coating, with the edge surfaces of the steel sheets acting as the origin. As a result, when using hot-dip Al—Zn alloy coated steel sheets in the field of building materials, edge creeps significantly larger than when using hot-dip Zn alloy coating would be generated and the anti-corrosion property after coating would be inferior.
In order to solve the above problems, for example, PTL 1 (JP2002012959A) discloses a hot-dip Al—Zn alloy coated steel sheet obtained by adding Mg, and further Sn or the like to the coating composition, to form Mg compounds such as Mg2Si, MgZn2, Mg2Sn in the hot-dip coating layer to inhibit generation of red rust from the edge surfaces of the steel sheet.
However, when applying a coating film on the hot-dip Al—Zn alloy coated steel sheet disclosed in PTL 1, the problem relating to the anti-corrosion property of when the coating film is later damaged (anti-corrosion property after coating) could not be resolved.
Further, there are cases where hot-dip Al—Zn alloy coated steel sheets are used in the field of building materials or consumer electronics without applying a coating film thereon. Particularly when used as wall materials, back plates of consumer electronics and the like, the surface of the coated steel sheet is exposed to be seen and therefore good appearance quality is required. Appearance quality is determined mainly by the presence of defects such as adhesion of foreign matters, bare spots, flaws as well as variation in patterns and colors. Patterns and colors are qualities strongly required when hot-dip Al—Zn alloy coated steel sheets are used without coating. Therefore, not all hot-dip Al—Zn alloy coated steel sheets used as film coated steel sheets are applicable to applications where the hot-dip Al—Zn alloy coated steel sheets are used without a coating film (wall materials, back plates of consumer electronics and the like), and there was also a demand for further improvement in appearance quality.
Further, with hot-dip Al—Zn alloy coated steel sheets, there were cases where the hot-dip coated surface would gradually turn into a black color (blackening), depending on the composition of the hot-dip coating. For example, even with the hot-dip Al—Zn alloy coated steel sheets containing Sn disclosed in PTL 1, there are cases where blackening occurs. As described above, not all hot-dip Al—Zn alloy coated steel sheets could be used in applications where the sheets are used without a coating film.